This invention relates to solder masks used in the manufacture of printed circuit boards. More particularly, it relates to dry film solder mask compositions curable by exposure to U.V. radiation.
Dry film solder masks provide protection to the delicate circuitry on printed circuit boards during processing steps, such as etching and soldering, as well as providing continued protection from environmental stresses throughout the life of the circuit board. These masks must be resistant to solder compositions, as well as etching fluids such as methylene chloride, and also withstand environmental stresses such as changes in temperature and humidity, while remaining flexible enough to withstand mechanical stresses without cracking or peeling.
A dry film solder mask composition is, generally, applied to a circuit board, partially cured in the desired areas by exposure imagewise (through a phototool or negative) to ultraviolet radiation, and, after the unexposed composition is removed from the board, the partially cured mask is then fully cured by further exposure to U.V. radiation and heating. The art has taught that the final heating step, also called a thermal cure, is necessary to completely cure the mask so as to adequately protect the circuit board.